1. Field of the Invention
The present invention relates to an image pickup apparatus and more particularly to an image pickup apparatus using solid-state image sensors.
2. Description of the Prior Art
It is necessary to drive solid-state image sensors (e.g., CCD) in synchro with the television field period if they are used for television image sensing. For example, a television system in conformity with the NTSC system has its field period of 1/60 sec. Therefore, where frame transfer type image sensors are used, it is necessary to transfer signals (charge) from the image pickup or sensing area to the image storage area at the period of 1/60 sec and drive the image storage area during about 1/60 sec in order to read the image storage signal (charge) from the image storage area.
Apart from the above, it is also necessary to suppress the outputs of the sensors and the image signals obtained by processing the sensor outputs within a certain stable range regardless of a wide brightness variation of the object to be sensed. In this respect, the quantity of incident light to the image pickup area has been adjusted heretofore by using such as a stop, or the image signal level has been adjusted heretofore by inserting an AGC amplifier in the output processing system.
However, in case the object to be sensed has an extremely high brightness, adjustment of the signal level often becomes unsatisfactory if such a stop or an AGC amplifier only is used.
Furthermore, if an object to be sensed moves at a high speed, undesirable so-called image-running or image-fluctuation may occur while displaying on a display electrical signals obtained from the image pickup area. This results from the fact that the light reception time at the image pickup area, under drive in conformity with the television field period, is about 1/60 sec.
The above problems arise from the fact that the image sensors are driven in accordance with the television field period. In order to solve this, it is known to control the light reception time by using a mechanical shutter. Use of a mechanical shutter again leads to the problems that the image pickup apparatus becomes bulky, the construction and control system become complicated, and the cost for the apparatus becomes expensive.
In order to solve such problems, it is known in the art to provide an unnecessary charge removal function for removal of unnecessary charge from the image pickup area. Of the known drive methods for a frame transfer type solid-state image sensor provided with an unnecessary charge removal area, one method realizing such unnecessary charge removal function has been proposed, wherein at a certain time instant during the charge storage period at the image sensing area, the image sensor is driven so as to remove the storage charge stored upto such time instant in the image pickup area via the unnecessary charge removal area; and whereby a mechanical means such as a shutter is not needed and a shorter charge storage time than the field period can be obtained.
For example, a method using such unnecessary charge removal function is disclosed in the Official Gazette of Japanese Patent Publication No. 27712/1983 corresponding to U.S. Pat. No. 4,245,164 issued on Jan. 13, 1981. According to the method, an elimination electrode or overflow drain (hereinafter called top drain) is provided on the opposite side of the image pickup area relative to the image storage area of a known frame transfer type solid-state image sensor. The storage charge in the image pickup area at a certain time instant during the charge storage period is transferred in the opposite direction (i.e., the direction opposite to that of readout) toward the top drain and collected at the top drain where the image storage charge is eliminated.
The assignee of Japanese Patent Application No. 61098/1983 corresponding to U.S. Ser. No. 596,404 filed on Apr. 3, 1984 has proposed and disclosed therein a method realizing unnecessary charge removal function. In particular, in the method of driving a known frame transfer type solid-state image sensor provided with a blooming preventive overflow drain serving as an excessive charge removal area in the image pickup area at a certain time instant during the charge storage period at the image pickup area, the image pickup area is driven independently from the image storage area so that the storage charges in the image pickup area are collected at the boundary between the image pickup and storage areas to remove them via the overflow drain positioned at the boundary.
Alternatively, as disclosed in the Official Gazette of Japanese Unexamined Publication No. 18064/1980 corresponding to U.S. Pat. No. 4,328,432 issued on Jan. 14, 1981, there is known a construction of a frame transfer type or interline type image sensor, wherein a blooming preventive area for removing excessive charge by means of the charge recombination method is provided in the image pickup area. With such construction, it is possible to remove excessive charge by driving the blooming preventive area at a proper timing during the charge storage period at the image pickup area.
Still further, it is well known that an image pickup apparatus using a solid-state image sensor, similar a conventional image tube, requires a clamping circuit provided in the image signal processing system. The clamping circuit is provided for maintaining the DC level of an image signal at a constant level, for example, for maintaining the DC level of each horizontal scan signal at a certain black level. To this end, a light shield portion (hereinafter referred to as optical black) is provided in the image pickup area of the image sensor at a position corresponding to the head portion of each horizontal scan signal. Thus, it is arranged to clamp the signal level obtained by the optical black to a constant level (reference black level or optical black level).
As described in the foregoing, when an opposite directional and vertical transfer pulse (in case of the Official Gazette of Japanese Patent Publication No. 27712/1983) or a normal directional and vertical transfer pulse (in case of the Official Gazette of Japanese Patent Application No. 61098/1983) is applied to the image pickup area for removal of the unnecessary charge during charge storage at the image pickup area, or a drive pulse (in case of the Official Gazette of Japanese Unexamined Patent Publication No. 18064/1980) is applied to the recombination blooming preventive area in the image pickup area for removal of the excessive charge. At those time instants, reading signal charge from the storage area and its processing are being performed (the above pulses are generally called charge clear pulse hereinafter where applicable). As a result, as the charge clear pulse is applied, the signal charge is superposed with the charge clear pulse as a noise. Particularly while a clamping operation is being performed by reading the signal charge from the optical black, there arises a problem that the DC level of an image signal charge other than that from the optical black changes to a large degree because of the superposed noises. Such a problem will be described in a concrete way with reference to the accompanying drawings.
FIG. 1 shows a relationship between the above-noted charge clear pulse and a one field (1V) image signal obtained by reading signals from a solid-state image sensor. The image signal V-out shown in FIG. 1A is obtained by reading signals from the sensor, during such reading period the charge for the next image signal being accumulated as well known in the art in the image pickup area. By applying a charge clear pulse .phi. cl shown in FIG. 1B to the image pickup area at a certain timing while reading signals from the sensor or while accumulating charge in the image pickup area, substantially all of the previously stored charge is cleared in case of the drive method disclosed in the above-described Official Gazette of Japanese Patent Publication No. 27712/1983 or Japanese Patent Application No. 61098/1983. The effective charge storage period becomes t which is shorter than 1V period. In case of the construction as disclosed in the Official Gazette of Japanese Patent Unexamined Publication No. 18064/1980, excessive charge is to be removed. In the figure, references Sv and Sh denote the periods during which vertical and horizontal pulses are applied, respectively.
FIG. 2 shows a relationship between a one horizontal line (1H) image signal, clamp pulse for the clamping circuit, and charge clear pulse. As shown in FIG. 2A, the reference black level (optical black level) OB is obtained during a predetermined time period immediately after each horizontal synchro period Sh in the image signal V-out, by virtue of the construction of the image pickup area of the image sensor which will be described later. Also as will be described later, the clamping circuit in the image signal processing system is arranged to clamp the reference black level upon reception of a clamp pulse .phi. cp. FIG. 2C shows the above-described charge clear pulse .phi. cl and is applied over several Hs as understood from FIGS. 1A and 1B.
As shown in FIG. 3A which shows a portion of FIG. 2A magnified in time scale, noises CN are interposed on the image signal V-out due to the charge clear pulses .phi. cl. Therefore, as a black level clamping is performed during the period of a clamp pulse .phi. cp shown in FIG. 3C, the clamp level shifts due to such noises CN so that a correct DC restoration of an image signal becomes impossible.
In order to eliminate such disadvantage, the assignee of U.S. Ser. No. 602,731 filed on Apr. 23, 1984, now abandoned, has proposed a technique to stop application of the charge transfer pulses .phi. cl to the image sensor during the period the clamp pulse .phi. cp is being outputted.
However, in case the energy of the charge transfer pulse .phi. cl is relatively large, even if the application of the charge transfer pulses .phi. cl is stopped at the time when a clamp pulse .phi. cp is outputted, there is a possibility for such stoppage to adversely affect the image signal so that a sufficient effect cannot be expected.